1. Field
This invention pertains generally to nuclear reactor monitoring systems and more particularly to an in-core power distribution monitor.
2. Related Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated in heat exchange relationship with a secondary circuit for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump, and a system of pipes which are connected to the vessel form a loop of the primary side.
For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pump 16, through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
An exemplary reactor design is shown in more detail in FIG. 2. In addition to the core 14 comprised of the plurality of parallel, vertical, co-extending fuel assemblies 22, for purposes of this description, the other vessel internal structures can be divided into the lower internals 24 and the upper internals 26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in FIG. 2), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 2, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the vessel and the core barrel 32, is turned 180° in a lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, a lower core support plate having the same elevation as 37. The coolant flow through the core and surrounding area 38 is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate 40 and upwardly in a plurality of perforations 42. The coolant then flows upwardly and radially to one or more outlet nozzles 44.
The upper internals 26 can be supported from the vessel or the vessel head and include an upper support assembly 46. Loads are transmitted between the upper support assembly 46 and the upper core plate 40, primarily by a plurality of support columns 48. Essentially, each of the support columns is aligned above a selected fuel assembly 22 and perforations 42 in the upper core plate 40.
Rectilinearly moveable control rods 28, which typically include a drive shaft 50 and a spider assembly 52 of neutron poison rods, are guided through the upper internals 26 and into aligned fuel assemblies 22 by control rod guide tubes 54. The guide tubes are fixedly joined to the upper support assembly 46 and the top of the upper core plate 40. The support column 48 arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability.
FIG. 3 is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character 22. The fuel assembly 22 is typically used in a pressurized water reactor and has a structural skeleton which, at its lower end, includes a bottom nozzle 58. The bottom nozzle 58 supports the fuel assembly 22 on the lower core plate 36 in the core region of the nuclear reactor. In addition to the bottom nozzle 58, the structural skeleton of the fuel assembly 22 also includes a top nozzle 62 at its upper end and number of guide tubes or thimbles 84 which align with the guide tubes 54 in the upper internals. The guide tubes or thimbles 84 extend longitudinally between the bottom and top nozzles 58 and 62 and at opposite ends are rigidly attached thereto.
The fuel assembly 22 further includes a plurality of transverse grids 64 axially spaced along and mounted to the guide thimbles 84 and an organized array of elongated fuel rods 66 transversely spaced and supported by the grid 64. The grids 64 conventionally formed from an array of orthogonal straps that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells, many of which support the fuel rods 66 in a transverse, spaced relationship with each other. The remaining cells are occupied by the control rod guide thimbles 84 and an instrument thimble 68. As shown in FIG. 3, the instrument tube or thimble 68 is located in the center of the fuel assembly and extends between and is captured by the bottom and top nozzles 58 and 62. With such an arrangement of parts, fuel assembly 22 forms an integral unit capable of being conveniently handled without damaging the assembly of parts.
As mentioned above, the fuel rods 66 in the array thereof in the assembly 22 are held in spaced relationship with one another by the grids 64 spaced along the fuel assembly length. Each fuel rod 66 includes a plurality of nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74. The pellets 70 are maintained in a stack by plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The pellets 70, composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system.
To control the fission process, a number of control rods 78 are reciprocally moveable in the guide thimbles 84 located at predetermined positions in the fuel assembly 22. Specifically, a rod cluster control rod mechanism 80, positioned above the top nozzle 62 supports a plurality of the control rods 78. The control mechanism has an internally threaded cylindrical hub member 82 with a plurality of radially extending flukes or arms 52 that form the spider previously noted with regard to FIG. 2. Each arm 52 is interconnected to a control rod 78 such that the control rod mechanism is operable to move the control rods vertically in the guide thimbles 84 to thereby control the fission process in the fuel assembly 22, under the motive power of a control rod drive shaft 50 which is coupled to the control rod hub 80 all in a well-known manner.
Movement of the control rods is used to shape the axial and radial power distribution to maintain the peak fuel rod cladding temperatures within acceptable limits. To monitor this process, and to provide information for the control and protection systems, in-core neutron monitors for monitoring the neutron radiation and thermocouples for monitoring the core exit temperature are provided in a number of the fuel assemblies, within the instrument thimbles 68. The signal leads from these sensors have typically been routed, at first through the bottom of the reactor vessel and more recently through the upper internals, exiting through the reactor vessel head, to a control center. However, the top mounted instrumentation complicates the refueling process, because these sensors have to be removed from the core before the fuel assemblies can be accessed for relocation or replacement. The withdrawal of the instrumentation from the core and the later replacement of the instrumentation after the fuel assemblies in the core have been reconfigured adds significantly to the time required to complete the refueling process which typically is on the critical path of an outage. Conserving outage time to a utility operator is a critical objective, because of the high cost of replacement power incurred during an outage.
Accordingly, it is an object of this invention to provide a mechanism for monitoring the axial and radial distribution of the power within the core that will not be required to be removed during a refueling outage,
Additionally, it is an object of this invention to provide such a sensor arrangement that can be installed in a majority if not all of the fuel assemblies without creating an obstruction to coolant flow within the upper internals,
Further, it is an object of this invention to supply such a sensor system that can be manufactured as an integral part of the fuel assembly.